1. Field of the Invention
The present invention relates to a zoom lens suitable for a still camera, video camera, a digital still camera, and the like and, more particularly, to a zoom lens suitable for a film still camera, video camera, digital camera, or the like, which has three lens units including a lens unit with a negative optical power (in the specification, the optical power is equal to the reciprocal of a focal length) preceding other types of lens units, in particular, and optimizes the lens arrangement of these lens units to reduce the size of the overall lens system.
2. Related Background Art
With recent advances in the performance of cameras (optical devices) such as video cameras, digital cameras, and electronic still cameras using photoelectric conversion elements such as solid-stage image pickup elements, optical systems used for them have been required to attain high optical performance and miniaturization.
In a camera of this type, various optical members such as a low-pass filter and color correction filter are required to be located between the final lens portion and an image pickup element. Therefore, as an optical system used for this purpose, a lens system is required to have an optically relatively long backfocus. In addition, in a camera using a color image pickup element, to avoid color shading, an optical system used for this camera is required to exhibit good telecentricity on the image surface side.
Conventionally, various lenses of so-called short zoom type lenses have been proposed, each of which is comprised of two lens units, i.e., a first unit having a negative optical power and a second unit having a positive optical power, and designed to perform zooming or magnification changing by changing the lens distance of both units. In such a short zoom type optical system, the system performs zooming by moving the second unit having a positive optical power and corrects the image point according to zooming by moving the first unit having a negative optical power. In the lens configuration constituted by these two lens units, the zoom ratio is about 2×. Three-unit zoom lenses are disclosed in Japanese Patent Publication No. 7-3507 (corresponding to U.S. Pat. No. 4,810,072), Japanese Patent Publication No. 6-40170 (corresponding to U.S. Pat. No. 4,647,160), and the like, each of which has a third unit having a negative or positive optical power on the image side to correct aberration due to a high zoom ratio, thereby realizing a compact structure as a whole while ensuring a higher zoom ratio.
Since these three-unit zoom lenses are mainly designed for 35-mm film photographs, it is hardly said that such a zoom lens realizes both a backfocus length required for an optical system using a solid-stage image pickup element and good telecentric characteristic.
Japanese Patent Application Laid-Open No. 63-135913 (corresponding to U.S. Pat. No. 4,838,666), Japanese Patent Application Laid-Open No. 7-261083, and the like disclose three-unit zoom lens systems, each of which is comprised of three lens units respectively having negative, positive, and positive optical powers and satisfies both the backfocus requirement and the telecentric characteristic requirement. An optical system is also disclosed in Japanese Patent Application Laid-Open No. 3-288113 (corresponding to U.S. Pat. No. 5,270,863), which performs zooming by fixing the first unit having a negative optical power, of a three-unit zoom lens having lens units with negative, positive, and positive optical powers, and moving the second and third units having positive optical powers.
The present applicant has disclosed an image taking lens having a three-unit configuration with negative, positive, and positive optical powers in Japanese Patent Application Laid-Open No. 2000-111798. According to this image taking lens, a zoom lens which has a zoom ratio of 2× or more and has realized a compact structure by minimizing the total length is implemented while a lens back long enough to insert a filter or the like on the image surface side is ensured and a telecentric characteristic required for a solid-state image pickup element is obtained.
U.S. Pat. No. 4,969,878 discloses a three-unit zoom lens having lenses with negative, positive, and positive optical powers sequentially arranged from the object side to the image side, in which the third unit reciprocates along the optical axis with a convex locus on the object side in zooming.
In the three-unit zoom lenses disclosed in Japanese Patent Application Laid-Open No. 63-135913, Japanese Patent Application Laid-open No. 7-261083, and Japanese Patent Application Laid-Open No. 3-288113, the number of lenses constituting each lens unit is relatively large, and hence the total lens length tends to be long.
In the optical system disclosed in Japanese Patent Application Laid-Open No. 7-261083, since focusing on a near object is performed by moving the first unit having a negative optical power while fixing the third unit having a positive optical power, the mechanical structure tends to be complicated owing to the movements of lens units in zooming as well.
U.S. Pat. No. 4,999,007 discloses a three-unit zoom lens with negative, positive, and positive optical powers, in which each of the first and second units is formed by a single lens.
However, the zoom lens disclosed in this reference is relatively long in total lens length at the wide angle end. In addition, since the first unit is greatly spaced apart from the aperture stop at the wide angle end, the incident height of an off-axis ray is high, and the diameter of the lens forming the first unit becomes large. The size of the overall lens system therefore tends to be large. Furthermore, since each of the first and second units is formed by one lens, aberration correction is insufficient in each lens unit. Variations in magnification chromatic aberration on zooming, in particular, tend to occur in the first unit in which variations in height from an off-axis ray are high. Since the first unit is formed by one negative lens, aberration correction is not sufficiently performed within the lens unit. Variations in magnification chromatic aberration therefore tend to increase in the overall system.
A projection optical system having a three-unit configuration with negative, positive, and positive optical powers is disclosed in U.S. Pat. No. 4,824,223. In this optical system, the first unit is formed by one negative lens, and hence aberration correction in the lens unit is insufficient, and the zoom ratio is about 1.7×. That is, this optical system is not suitable for high zooming operation.
In addition, three-unit zoom lenses each having lenses with negative, positive, positive optical powers, sequentially arranged from the object side, and including a third unit comprised of a plurality of lenses including negative and positive lenses are disclosed in U.S. Pat. No. 4,838,666, Japanese Patent Application Laid-Open No. 62-200316, Japanese Patent Application Laid-Open No. 2-118509, U.S. Pat. Nos. 4,999,007, 5,835,287, Japanese Patent Application Laid-Open No. 5-173073 (corresponding to U.S. Pat. No. 5,434,710), and Japanese Patent Publication No. 60-42451.
According to U.S. Pat. No. 4,838,666, Japanese Patent Application Laid-Open No. 62-200316, Japanese Patent Application Laid-Open No. 2-118509, since the third unit is fixed during zooming, it tends to be difficult to maintain good performance throughout the entire zooming range in high zooming operation.
According to the U.S. Pat. Nos. 4,999,007 and 5,835,287, since the number of constituent lenses of the first and second units is as small as one or two, it is difficult to satisfy both the high zoom ratio requirement and the performance requirement. According to Japanese Patent Application Laid-Open No. 5-173073, since the third unit is constituted by positive and negative lenses spaced part from each other by an air gap, a deterioration in performance tends to occur due to relative decentering of these lenses.
According to Japanese Patent Publication No. 60-42451, since the number of constituent lenses of the second unit is as large as 4 to 5, a problem arises in terms of miniaturization.